The present invention relates to geophones used in seismic exploration and more particularly to a method and apparatus for testing geophones for both their activity and their distortion and noise level. Geophones used in seismic exploration are electromagnetic devices consisting of a magnet, a spring-mounted coil and a case for protecting the internal parts and support them in their proper relationship. When the geophone is placed on the ground, any movement of the earth will move the magnet with respect to the coil and produce a voltage signal. The voltage produced by the coil will be linear as long as the magnetic field is uniform and the magnet is moved linearly in relation to the coil. The voltage will not be linear if the magnetic field is not uniform or the springs prevent the coil from moving linearly. In addition, a restriction on the movement of the coil will reduce the geophones output voltage.
Recent developments in seismic exploration have been directed to detecting anomalies in seismic signals which at times are related to hydrocarbon deposits. The techniques for detecting amplitude anomalies in seismic data have been referred to as bright spots and similar terms. It is obvious that if one desires to inspect seismic data for amplitude anomalies, the first requirement is, the geophones used must produce uniform signals having low distortion and noise. In the past, it had been the practice to test the geophones in a central facility for distortion and noise level and rely upon a test of the overall resistance of the geophones when they were placed in the field to indicate their operativeness. After the measurement of distortion and noise in the central facility, the geophones could be mishandled and damaged while their measured resistance remained unchanged. For example, if the springs used for mounting the coil are bent, it is possible for the coil to drag on the magnet and introduce distortion in the signal. This type of damage to the geophone is not discovered by measuring the resistance of the geophones after they are placed in their desired locations. Also, frequently the geophone is not planted in an upright position but at an angle and in extreme cases, on their sides so that the coil drags on the magnet and fails to produce a true signal. Again, this type of condition is not detected by measuring the resistance of the geophone circuit. The only fault detected by measuring the resistance of the geophones after they are planted is the presence of an open circuit either in the cable connections or in the geophone coils.
In a paper given at the annual meeting of the Society of Exploration Geophysicists in Dallas, Texas, November 10-14, 1974, O. A. Fredrikson, E. P. Meiners and E. L. Thomes discussed a geophone tester they developed. Their tester applied a constant current to displace the geophone and then measured maximum and minimum voltages produced and the time between the termination of the current pulse and the first zero crossover of the voltage signal. From these measurements, they provide expressions for calculating damping factor, frequency of mechanical resonance and relative sensitivity. The authors also describe a system for measuring impedance at natural frequency of the geophone. The impedance measurement uses a constant AC current and observes the voltage generated across the input as an indication of malfunctioning geophones.